Simplify the following expression: $t = \dfrac{6}{7y} - \dfrac{4}{9y}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $7y$ and $9y$ $\lcm(7y, 9y) = 63y$ $ t = \dfrac{9}{9} \cdot \dfrac{6}{7y} - \dfrac{7}{7} \cdot \dfrac{4}{9y} $ $t = \dfrac{54}{63y} - \dfrac{28}{63y}$ $t = \dfrac{54 -28}{63y}$ $t = \dfrac{26}{63y}$